Electron accelerator



Patented June 7, 1960 ELECTRON ACCELERATOR Richard F. Post, WalnutCreek, Califi, assignor to Applied Radiation Corporation, Walnut Creek,Calih, a corporation of California Filed Feb. 8, 1955, Ser. No. 486,865

Claims. (Cl. 315-542) The present invention relates to an improvedmethod and means for injecting electrons into and applying acceleratingenergy to a traveling-wave electron accelerator.

Conventional traveling-wave electron accelerators comprising a waveguidehaving spaced walls defining, in effect, successive resonant cavitiesare normally energized by a radio-frequency vacuum tube such as avelocity-modulated electron tube coupled thereto and have associatedtherewith an electron gun source admitting a beam of electrons into thewaveguide. The present invention provides method and means forenergizing an accelerator waveguide and injecting high voltage electronstherein in a single operation. The invention contemplates thesimultaneous production of radio-frequency energy and high voltageelectrons with a subsequent further acceleration of said electrons bythe radio-frequency energy in the form of a traveling voltage wave. Onemanner of carrying out this method is to couple the radio-frequencyenergy and electrons from a velocity-modulated electron tube into atraveling-wave linear accelerator in correct phase so that the energyexcites the accelerator waveguide to further accelerate the back phase(voltage-doubled) electrons injected.

The method and means of the present invention is herein illustrated anddescribed with respect to specific steps and structure in the interestof clarity, however, no limitations are intended or be inferredtherefrom, reference made to the appended claims for a precisedelineation of the scope of the invention.

The invention is illustrated in the accompanying drawing wherein:

Figure 1 is a schematic representation of a preferred embodiment of theelectron accelerating means of the invention;

Figure 2 is a schematic representation of alternate collector couplermeans;

Figure 3 is a section View taken at 3-3 of Figure 2; and

Figure 4 is an alternate embodiment of the collector coupler.

Considering now the invention in some detail as to the method thereof,it is contemplated that there will be produced a bunched beam ofelectrons. These electrons may be continuously emitted from anappropriate electron emitter such as ahot filament, for example, and theelectrons are accelerated by suitable electrical fields to a relativelyhigh potential and collimated into a beam. This beam of electrons isthen velocity modulated as by time varying electrical fields to producea bunched electron beam having periodic electron density variations. Thebunched electron beam is then employed to produce electromagnetic energyas by passing same successively through one or more resonant catchercavities whereby same are excited. There thus results a bunched electronbeam and electromagnetic radiation with the latter being employed tofurther accelerate a small portion of the bunched beam and return energythereto so that there is produced a resultant high voltage electronbeam.

There is produced in the first portions of. the above process a voltagedoubling action whereby certain relatively few electrons in the bunchedbeam attain substantially twice the velocity or voltage of the remainingelec trons, by virtue of passing through the catcher cavity 180 out ofphase with the phase of driven oscillations in said cavity. Thesevoltage-doubled electrons are herein chosen for further acceleration.Following production of the electromagnetic energy by the bunchedbeam','the

beam is collimated or otherwise operated upon to materially reduce thebeam current and as the main portion of the beam tends to defocus in theprocess of establish ing the electromagnetic energy, collimation of beamdiscriminates in favor of thevoltage-doubled electrons.

In addition to collimating the beam, the electromagnetic energy wavesare directed along the beam path for returning energy to the beam. Inorder; for the beam to be accelerated by the electromagnetic waves 'thephase of the latter is varied with respect to the voltage-doubledelectron beam pulses, or vice versa, and in this respect certainalternative steps are possible. In order to obtain the proper phaserelation, either the phase of the electromagnetic Waves may be advancedwith respect to the beam pulses or it may be retarded with respectthereto. Advancing the phase of the electromagnetic waves may beaccomplished by passing both the energy waves and electron beam througha transmission line wherein the wave velocity is substantially equal toor greater than the speed of light and the electron beam velocity ismuch less so that in a predetermined distance a desired phase advance isachieved. Retarding of the energy wave phase may be accomplished bydirecting both energy waves and the beam through a slow wave structurewherein the wave velocity is reduced to less than that of'the beam andwherein a predetermined distance of travel shifts the relative phase tothe desired relationship. Although'both of the above alternatives arefeasible, a minimum length of travel of beam and wave results eitherfrom the slow wave propagation or the propagation of the wave at avelocity greater than that of light for in both cases a maximumizedvelocity difference between wave and beam is attained. Certainadvantages will be seen to attach to the minimization of distancerequired to accomplish this step of the method. 7

Acceleration of the voltage doubled portion of the bunched beam isaccomplished by establishing traveling voltage waves with theelectromagnetic waves originally produced by the beam and passing thebeam therethrough in proper phase to receive acceleration therefrom. Asto the proper phase relation same is produced in the above-noted mannerso that the voltage waves established do accelerate the voltage-doubledelectrons of the beam. There is produced by the above process a highvoltage electron beam of periodically varying electron density. Theoriginal electron beam produced in the process will be seen to haveactually supplied energy to a small portion of itself to drive same to ahigh voltage and this is accomplished by producing electromagneticenergy from the beam and then feeding same back into a selected portionthereof. Various structural arrangements may be employed to carry outthe described method and referring to Figure 1 of the drawing whereinone preferred means is illustrated, there is shown a velocity-modulatedelectron tube 11 axially aligned with the waveguide 12 of atraveling-wave linear accelerator 13 and communicating therewith througha cylinder 14 disposed between same. The electron tube 11 may consist ofa conventional electron-beam radio-frequency tube such as klystron andincluding an electron emitter 16 having a filament and heating meanstherefor to emit electrons within the tube. Electrons are acceleratedfrom the emitter 16 by an anode 17 which may be formed as a transversetube Y the collector-coupler.

time d thence through a buncher cavity 18 aligned with v emitter andadapted for excitation by an externally time-varying voltage of a radiofrequency, for example. The electron beam is velocity modulated byabove, the proper choice of length of the collector-coupler 22 willplace the voltage-doubled electrons in proper phase relation to thecavity excitation so that only bunches of voltage-doubled electrons areaccelerated thereby. The

this yoltage in the buncher cavity 13 in-a conventional manners6 thatthe electron beam in traversing a relatively field tree. space beyondthe cavity 18 becomes bunched h pe iodic variations in electron density:Following ncher cavity 18 is asecond cavity 2 lconnected .toth first'jbyi an elongated passage 19 through which the eleetron beam travels.V'Fhe secondjcavity '21 is designed to resonate at the frequency of thebunched beam so that there is thus produced in a conventional manner aresonant energy exchange within the cavity 21 wherebyelectromagneticenergy is produced therein.

The above-described' elements and operation are conventional intubessuch as a 'klystron, however, the electron beam is conventionallyintercepted by a collector beyond the output cavity 21 with theradio-frequency energy developed in the tubebeing fed into otherapparatus asby coupling loops or the like for use therein. The.presentiinvention removes both the electron'beam and if:idjo-freq-uencyenergy from the tube and forthis purpo'sej provides'a collector-coupler22 in, axial align mentwith the tube 11 communicating with the outputcavity21thereof... This collector-coupler 22 includes the cylinder 14and an inner member 23 shown as a cylinder or solid of revolution havingan axial passage therethrough similar to a reversed venturi with adiameter decreasing with separation from the cavity 21 and having anenlarged opening at the exit-thereof, as shown. The member 23 may bemounted as by a pair of quarterwave stubs 2'4 wit hinthecylinder 14 tominimize insulation problems as shown in Figure. l. A small openingtherethrough will .beseent to intercept and collect a large portion ofthe electron beam from the electron tube 11. 'Within the 11 a certainsmall portion of the electrons inthe beam experience an additional.acceleration to substantial- -ly twice the voltage of the restof thebeam electrons by virtueof their phase'being such as to be acceleratedin V the outputcavity 21 rather than decelerated. These electrons arecalled volt-agedoubled electrons; It is these ;vo ltage doubledelectrons which are preferentially chosen inftheprsent invention foradditional acceleration, since I itl canibeshown that an increase ininjection voltage increasesithe efiiciency of, and simplifies the designer,-

] trave ing Wave accelerators. V 1 Within the col1ector-coupler22 ofFigure l the radioirequency energy from the tube 11 travels at the speedof;;-light,between the cylinder 14"and member 23'while.

even .the voltage-doubled electrons travelingrthrough the couplingtheenergy waves advance in phase exactly the proper amount in relationto thebunches of voltagevdoubled electrons asrnoted below. Within theaccelerator by the collectorcouplerfn. There is thus established '1 inthe accelerator 12 a travelingwavereceiving its energy,

from the tube 11 and giving up energy to electrons passing@t-herethrough ,in proper phase relation thereto. As notedcollector-coupler 22 thus acts as'a collector for the electron beam ofthe tube 11 whileyet admitting passage of a portion of said beamtherethrough and also couples the tube energy into thewaveguide ,1 3-for exciting the cavities 27 thereof. V V r The portion of the electronbeam accelerated in the waveguide 12 maybe utilized exteriorly of'theapparatus by the provision of'a window 28'at the closed end of thewaveguide on the axisdhereof. The window 28 is pervious to theaccelerated electron beam and yet seals the system to maintain a vacuumtherein which is advantageous in the operation thereof.

With regard to the variation in phase of the electromagnetic waves andthe; voltage-doubled electrons of the radians) with respect to vthephaseoi the voltage-doubled electrons. With the. above described coaxialcollectorcoupler .of Figure l having 3a length L, the rfollowing Vwherein:

relationshipis attained ,for' 180 phase shi-ftin same:

2TB 21rL V an; Ber 7 k =freespace wavelength at operating frequencyBe=velocity of electron beam relative to free space velocity of light(0) ng=phase velocity of wave relative to free space velocity of light(0) Solving this relationship for L gives:

'4 My 2 rig-5 The phase velocity of the wave is substantially. equal tothe free space velocity of light so that 'fig=1 and in a typical case of300 thousandelec't'ron volts, fle=0. 8 so that substituting r in theabove relationship, 7

and in a typical velocity modulated electron tube, as a.klystron,;wherein the, wave. length is, 10.57 centimeters,

L =21 centimeters.

Although the above determined :lengthof. the cellectorcoupler isnotexcessive it, may be reduced by alternative structure suchas shown inFigures 2 and 3.

. Theialternative embodiment ofFig'ures '2 and '3 illustratesacollec'tor-coupler 31 including, a cylinder 14 conlar in structure tothe like numbered member of Figure l faxial passagein member-23 have amuch lower velocity soJthatthe, energy waves advance in phase withrespect -to-the bunched. electrons of the beam in passing through Bychoosing the proper. lengthof nected between the; output cavity-2:1 of.a-tube' 11, and the waveguide 12 of the accelerator 13. Coaxiallywithin this cylinder 14 is mounted'a cylindricalrnember 23 simibutshorter, as noted below. In this embodiment a fundamental coaxial, modeat eachend of the coupler is transformed in the central part of thecoupler to a special waveguide mode. Thus ateachendthe coupler 31 andsurrounding cylinder 14 are symmetrical so-that the electric'fiieldofthe'transmitted electromagnetic waves is directed radially outwardfrom the coupler axis and uniform about the member 23; This coaxial modeof transmission becauseof its 'axialxsymmetryispartic'ularly desirable.for coupling out of 'thetcavity 21 and into the accelerator 13 Thecentral portionof the coupler 31 is' supported by: a wall orupstandingmember 32,.as shown in Figure 2 and 3, paralleltothe lcoupler'axis'andbelow 'same between the cylinder 14 and member- 23; This thereby tobecome circumferential wherein the wave ve locity exceeds that of light.Small vertical posts 33 are disposed upon the cylinder 14 below themember 23 at each end of the wall 32 thereof for impedance matchingbetween the coaxial and waveguide modes and these posts areconventionally termed matching posts. While the aforesaid matching postsserve to illustrate an embodiment of the invention, various other meansfor accomplishing the same results will suggest themselves to thoseskilled in the art.

In the embodiment of Figures 2 and 3 wherein the wave velocity in thecenter section coupler exceeds the speed of light the length L of thecenter section having the wall 32 attached thereto may be determinedfrom the relation:

section is determined by the boundary conditions for the transmissionmode which may be stated as:

sin (K p)= or K =g wherein: p=mean circumferential dimension of thecoupler and K +K =K where Solving this for Bg gives and in a typicalcase wherein the diameter of the member 23 is 2.5 centimeters and thecylinder diameter is 3.5 centimeters, p=7.0 centimeters, so that with A=10.5 centimeters and 52:0.8, as above, g=1.60.

Thus, substituting in the above equation for L gives L=0.8)\ or 8.4centimeters. This dimension of 8.4 will be seen to be materially lessthan the length of 21 centimeters calculated above for a typicalstructure of the embodiment of Figure 1 and even with the addition tothis of the end sections of the coupler the resultant length issubstantially less than that of the coupler of Figure 1.

One further embodiment of the coupler is herein illustrated in Figure 4wherein a member 23 substantially identical to that of Figure 1 exceptfor size is mounted as by quarter wave length stubs 24 in a cylinder 14between the tube 11 and accelerator 13. In this instance a periodicstructure is formed within the coupler 34 formed by the above mentionedelements so as to produce a periodically loaded coaxial section to delaythe phase of the wave with respect to the electrons. Such a structure iswell known for its properties of slow wave propagation and in theillustrated embodiment there are provided upon the coupler a pluralityof radial flanges or discs 36 spaced evenly along the coupler upon theexterior surface thereof. As the wave is propagated between the cylinder14 and central member 23, the discs 36 form such a periodic structure asto materially reduce the wave velocity. Within practical limits the wavevelocity may be made as small as desired and, for example, a value offig=0.5, wherein fig here again equals the phase velocity of the waverelative to the velocity or" light, may be achieved with a ratio of discto member 23 diameter of about 1.5 to 1 and a ratio of cylinder tomember 23 diameter of about 1.7 to 1. The relationship noted in relationto the case of Figures 1 and 2 is here again ap- 21rL 21rL A BeBg) B sBy and 2 5 -1 9 Substituting 52:05 and Bg=0.8 in the above relationgives L=0.67)\ so that with A =l0.5 centimeters, L=7 centimeters. Inthis case, as in that of Figure 2, L is taken as the length of thecenter periodic section of the coupler 34 so that an addition is to bemade thereto of the unloaded end sections to determine the overalllength.

There has been described above an improved method and means for electronaccelerationwherein but a single source is employed to produce a pulsedelectron beam and energy for accelerating same. With regard to thealternative embodiments of the coupler illustrated and described, sameare presented as illustrative only and particularly the calculationsherein included are to be taken only as examples and in no way limitingupon the scope of the invention which is precisely delineated in thefollowing claims.

What is claimed is:

1. An electron accelerator comprising a velocity modulated electrontube, a waveguide disposed in axial alignment with said electron tubeand having transverse partitions defining connected resonant cavities,and coupling means intermediate and interconnecting said tube andwaveguide with coaxial means therein directing radiofrequency energyfrom said tube into said waveguide and having a small opening thereinlimiting electrons transmitted from said tube into said waveguide, saidcoupling means also including means for varying the relative phase ofsaid radio-frequency energy and beam to provide substantially degreesphase difference therebetween.

2. Improved electron accelerating means comprising a traveling-wavelinear accelerator including a waveguide, a velocity modulated electrontube generating a bunched beam of electrons for producingradio-frequency energy, said electron tube and accelerator wave-guidedisposed in axial alignment, and coupling means connecting said tube andwaveguide for admitting to said waveguide a portion of the bunchedelectron beam of said tube and coupling said radio-frequency energy tosaid waveguide with an advance in the relative phase of saidradio-frequency energy whereby said waveguide is energized to acceleratesaid bunched electrons therein.

3. An improved ejector for an electron accelerator comprising a velocitymodulated electron tube including means producing a bunched electronbeam and defining a resonant cavity traversed by said beam whereinelectromagnetic waves are excited thereby, and coupling meanscommunicating with said tube at said cavity in axial alignment with theelectron beam thereof, said coupling means defining coaxial openingstherethrough for passing said electromagnetic waves and a portion ofsaid electron beam in predetermined shifted phase relationship andadapted for connection to an electron accelerator to inject thereinelectromagnetic waves and a bunched electron beam.

4. Improved coupling means for coupling a velocitymodulated electrontube to a traveling-wave linear electron accelerator acceleratingWaveguide comprising an elongated tube adapted for axially alignedcommunicable connection between the output cavity of'saidvelocitymodulated tube and the input of said waveguide, and a couplingcylinder disposed coaxially within said elongated tube and having a beamcollimating aperture therethrough for transmitting a collimated bunchedelectron beam through the aperture and electromagnetic waves through theannulus between said cylinder and elongated tube from the output cavityof the tube to the input of the Waveguide.

5. Improved coupling means as claimed in claim 4 further defined by saidcoupling cylinder having smoothly curving convex inner walls taperingradially inward away 7 from the inletend thereoffor collecting themajority of electrons of an entering'electron beam and passing onl asmall percentage thereof. a

6. In an improved injector for an electron accelerator including aperiodically loaded accelerating waveguide,

, the combination oomprisingaklystron' having an openended outputcavity,a cylinder connected in axial alignment with said klystron in unimpededcommunication with the output'cavity thereof and adapted forcommunicable-connection to theaccelerating Waveguide of an elec- ;,tronlinear accelerator, and an inner coupling cylinder disposed in saidcylinder, said inner coupling cylinder having" a passage therethrough'tapered radially inward for collimating an electron beam from saidklystron' to pass'only a small portion thereof and having a lengthsufficient to advance the relative phase of electromagnetic Wave energyradiated from the klystron through the annulus between said innercoupling cylinder and outer .cylinder a predetermined amount over thatof the electron beam passing through said passage.

7. An electron accelerator comprising a velocity modulated electrontubeproducing a bunched electron beam and resonant electromagneticenergy, a traveling wave linear accelerator having a waveguide, andmeans coupling saidtube and waveguide in alignment and providingdirection communication for coupling the electromagther defined by saidcoupling means having a central aperture of 'small diameter collimatingsaid electron'beam to intercept all but the central core thereof andhaving an elongated waveguide structure wherein the relative phase ofthe energy and'electron beam varies in propor-' tion to the lengththereof-for establishing such a phase relationship that 'said'voltagewaves accelerate predetermined bunches of electrons in said beam. 7

' further defined by said couplerhavinga length sufficient 9. Anelectron accelerator comprising a velocity-moda lated electron tubeproducing resonant electromagnetic energy with a bunched electron-beam,a traveling wave electron accelerator structure'axially aligned with theelectron beam of said tube at the end thereof, and a couplercommunicating between saidtubeand said .ac-

. celerator structure with; said-coupler including 'a coaxial member forcoupling electromagnetic energy from said tube into said acceleratorstructure anddefining an axial aperture for collimating the electronbeam from said tube to substantially original. cross section prior toentry ,of sameinto said accelerator structure: V g 1 '10. An electronaccelerator comprising tube 'means es-, 'tablishing resonantelectromagnetic energy with .a bunched electron beam, coupling meanscommunicating "with said tube means in line withthe ,electronbeam ofsame for. passage of said beam. therethrough and including aperiodically loaded waveguide structure coaxial section whereinsaidelectromagnetic energyis'transmitted at a low propagation velocityso that the phase of said electron beam advances over that of thevtransmitted energy, and a plurality of communicating resonant cavitiesof successively increasing volume'connectedto said coupling meanswherein said electromagnetic energy estab- V lishes traveling voltagewaves for accelerating portions of said electron beampassingtherethrough.

' to retard the phase of the electromagneticenergy one hundred andeighty degrees relative to the phase of said bunched electron beam. J 11 12. In'an injector for an ele'ctr onaccelerator' including a pluralityof communicating resonant cavities, the'co'nrbination comprising meansproducing a bu'nched electron beam, means defining a cavity 'resonantzit-the frequency of said beam bunches and disposed for beam traversetherethrough whereby resonant electromagnetic energy waves areestablished insaid cavity, and couplingmeans admitting a collimatedportion of. said electron beam intoisaid cavities for passagetherethrough, said coupling means including a coaxial section couplingsaid electromagnetic energy into-said cavities .for exciting same andincluding axially spaced radial disc loading said section for reducingWave propagation velocity therein whereby V bunches of said beam. 1

13. An electron accelerator comprising a velocity modulated electrontube including means establishing an electron beam, means bunching saidbeam'to produce spaced high density main beam bunches and intermediatesecondary beam bunches of lesser density, and means defining a cavitytraversed by said beam and resonant at the passage frequency of saidmain beam bunches for excitation thereby to produce electromagneticenergy waves accelerating secondary beam bunches; a periodic waveguidestructure defining consecutive cavities resonant at the frequency ofsaid energy Waves; and a coupler-collector communicating between saidelectron lube .and waveguide and including an; elongated coaxial sectioncoupling energy waves into said waveguide for energizing same and beamcollimating means collecting divergent low energy electrons of saidmainelectron bunches and passing high energy s'econdaryelectron bunches intosaid waveguide for acceleration therein. 7

14. An electron accelerator as claimed in claim 13 a further defined bysaid collector-coupler including means varying the relative phase ofoutput energy waves'to substantially correspond with the phase of saidsecondary electron bunches.

15. An electron accelerator as 13 further defined by saidcollector-coupler having a-predetermined field free length with meanscontrolling propag'ation velocity of said "energy waves'therethrough forsetting the relative phase of'exciting energy waves-and f electronbunches.

References Cited in thetile this patent UNITED STATES PATENTS;

2,595,698 Peter May 6; 1952 2,630,544 Tiley Mar. 3, 1953 "2,647,219.Touratonet a1. Iul'y'28, 1953 2,653,271 Woodyard Sept. 22, 19532,810,855 Miller at al. Oct. 22, 1957 2,813,996 6 Chodorow Nov. 19, 1957f FOREIGN PATENTS Y Great Britain 'Au'g:29, 1951

